1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus, and more particularly to an ultrasonic diagnostic apparatus capable of displaying tomographic images.
2. Description of the Related Art
An ultrasonic diagnostic apparatus enables the user to observe the inside of a subject by irradiating the subject with an ultrasonic wave, and analyzing information contained in the echo signal thereof. A commonly-used conventional ultrasonic diagnostic apparatus images the anatomy of a subject, in the form of a tomographic image, by converting the intensity of an echo signal to the brightness of the corresponding pixel. This enables the user to know the internal anatomy of the subject.
Generally, an ultrasonic wave attenuates while propagating through a subject. This generally decreases the intensity of the reflection wave obtained from inside the subject. Where a two-dimensional tomographic image is obtained by scanning a subject with an ultrasonic wave, the intensity of the reflection wave obtained through the scan may vary depending on, for example, the degree of contact between the subject and the probe. Such variation in the reflection intensity lowers the detection sensitivity in detecting a receive signal based on the reflection wave.
In order to correct such variations in the detection sensitivity, Japanese Laid-Open Patent Publication Nos. 2000-197637 and 2005-152422 disclose techniques for adjusting correction values in DGC (Depth Gain Control) for correcting the sensitivity in the depth direction, and LGC (Lateral Gain Control) for correcting the sensitivity in the scan (lateral) direction.
Specifically, Japanese Laid-Open Patent Publication No. 2000-197637 discloses a technique including: dividing an image frame into a regular grid of kernels; comparing the mean pixel intensity with the mean noise level predicted using a noise model for each kernel; selecting kernels in which the mean pixel intensity is greater than the mean noise level by a predetermined quantity and calculating the mean value of the mean pixel intensities of these kernels to thereby calculate the row/column mean of pixel intensity; and using the difference between the mean value and the reference value as the correction value. The publication also discloses performing a gain adjustment which will suppress the noise for each row/column in which the number of un-selected kernels is less than a critical threshold.
Japanese Laid-Open Patent Publication No. 2005-152422 discloses a technique of obtaining the mean signal intensity for each depth of the image, and obtaining a correction value as the difference between the reference value and the normalized mean value, which is obtained by normalizing the mean signal intensities. The publication also discloses weighting the correction value by setting the weight to 1 when the variance value of the signal intensity for each depth is greater than the reference value and setting the weight to less than 1 when the variance value is smaller than the reference value.
In the methods disclosed in Japanese Laid-Open Patent Publication Nos. 2000-197637 and 2005-152422, the pixel intensity and the variance value are used as index values for noise determination. With these methods, however, the pixel intensity lowers due to the attenuation of the transmitted wave deep inside the body, and it is difficult to accurately identify, for example, a non-noise image with a low variance value such as an image of the inside of a liver.
The methods disclosed in Japanese Laid-Open Patent Publication Nos. 2000-197637 and 2005-152422 are effective in equalizing the gain level and suppressing noise, but are not suitable for emphasizing an anatomical tissue component in the image, e.g., by increasing the gain level for the parts of interest, such as vessel walls in carotid diagnosis, relative to the gain level of other regions, thereby making the image easier to view.
Moreover, since the methods disclosed in Japanese Laid-Open Patent Publication Nos. 2000-197637 and 2005-152422 use DGC and LGC for correcting the gain of the image, the unit of correction is either by degrees of depth or by scanning lines. Therefore, in a case where the image includes an anatomical tissue component that extends two-dimensionally, such as the inside of the ventricle of the heart or in the vessels of the abdominal region, the gain correction varies in the scan direction or in the depth direction so that an area that should appear with even brightness may appear with uneven brightness.